1. Field of the Invention
This invention relates to the extraction and recovery of methane gas from coalbeds containing the same, and more particularly to a method by which dangerous methane gas trapped in coalbeds beneath the earth's surface can be released and recovered by the drilling of boreholes from the surface.
2. Description of the Prior Art
As long as underground coal mining has existed, the danger of explosions from methane gas has created a hazard. The methane has been extremely difficult to remove from shafts as coal seams are followed underground. As a result as mining continues, the concentration of the methane gas within the mine has caused explosions which result in loss of life and usually, inability to proceed further with the mine in which the explosion occurred. While a great deal of work has been carried out in an effort to minimize the hazards of methane in coal mines, occasional methane explosions still occur. At the present time, three control techniques are considered to be useful for the elimination of methane from coal mines. These control methods are: (1) controlled dilution with air or ventilation of the mine; (2) blocking or diverting the gas flow in the coalbed by the use of adequate seals; and (3) the removal of pure or diluted methane through the use of boreholes.
The most widely used of these techniques is the introduction of ventilation air to reduce the concentration of methane to a safe level in the mine. It should be noted that the danger of methane explosion becomes a problem only when the concentration reaches dangerous levels. While work has progressed in the use of seals to divert the gas flow and boreholes to remove it, the general tendency of mine operators to date has been to continue to use ventilation or controlled air dilution techniques in order to avoid build-up of the methane gas.
A particular problem with ventilation of the mine concerns dust suppression. Dust suppression is a problem of great concern in mining because the dust is in many cases explosive and also causes respiratory problems to miners. Where methane is a serious problem, ventilation must be conducted in large volumes, creating higher velocity and this in turn, increases dust hazards because more dust is forced into suspension as well as greatly increasing ventilation costs. The present invention not only reduces the presence of methane at the working surface by predraining the same but also reduces dust suspension by decreasing volumes of air needed for dilution of methane. It further provides safer working conditions at less cost for power of fan operation.
The most recent work in this area has involved the removal of pure or diluted methane through boreholes drilled into the mine in order to fracture the coal and recover the methane liberated by the fracture. To the present time two methods of drilling boreholes have been utilized. In one method, the boreholes are drilled vertically from the surface above the mine so that the bits intersect the coal in a vertical manner. However, this method has not been satisfactory as it does not intersect with the major natural fracture direction so that large amounts of the methane gas are not liberated by this technique. These borehole techniques have been depended upon to work independently of each other and hence their effectiveness has been limited. In some instances, water has been flooded into one hole in order to force gas to migrate to another hole but this also has been only partially successful, due to the impermeability of coal. Various discussions of methods for the removal of methane gas may be found in the United States Bureau of Mines Information Circular 1973 entitled "Methane Control in United States Coal Mines -- 1972", Information Circular 8600, United States Department of Interior.
Since the vertical borehole method has not been completely successful, substantial attention has been paid to the use of horizontal boreholes in order to intersect with the major natural fracture direction of the coal so that larger quantities of the methane gas will be released. However, the main drawback in drilling horizontal holes is that the drilling operation must take place within the mine and thus interferes with mining operations during drilling, risks possible blowouts of gas during drilling and/or piping operations, and makes collection of the gas a difficult operation. Thus, while the vertical drilling method for methane drainage is the lack of contact with the major, fracture system in the coal which results in low productivity wells requiring long periods of time to drain substantial volumes of methane, the horizontal method has also been unsuccessful because of the difficulty of carrying out the drilling operation.
It has also been proposed for example, at page 13, of the Bureau of Mines Information Circular 8600, identified above, to make use of socalled multi-purpose boreholes in order to effect methane drainage. A discussion of multi-purpose boreholes is also set forth in the publication "Coal Age", pages 50-52, Jan., 1973. The multi-purpose borehole is a system which uses a combination of vertical and horizontal boreholes. In this system, a ventilation shaft for a mine is drilled several years before the mining operation starts. Thereafter, horizontal holes are drilled from within the ventilation shafts. As can be appreciated, to drain the methane, use of this system requires long range planning as the mine must be planned long in advance. Moreover, the system is extremely costly to carry out.
In some of these systems, water may be infused into the mine or explosive charges selectively exploded in order to increase drainage of the methane from the mine. One technique of this type is set forth in U.S. Pat. No. 3,650,564.
Activity with respect to the recovery of methane gas from coal mines has particularly increased in recent years, not only to prevent explosions because of methane gas accumulation in the mine, but also in attempts to recover the methane gas to be used as a source of energy since methane, at least in pure form, is the same as natural gas which is used to heat and cool many homes and industries. Thus, there is a need in the art for methods by which methane gas can be liberated from the coal in large quantities and also recovered in a form which can be used as a source of energy. Accordingly, the present invention is concerned not only with minimizing the problem of explosions in mines where methane gas is found, but also in providing procedures for the recovery of valuable methane gas for use as a source of energy.